Emerging Ionic Polymers for CO2 Conversion to CyclicCarbonates: An Overview of Recent Developments*

Rabia Jamil,ALiliana C. Tome,

BDavid Mecerreyes,

C,D and

Debbie S. SilvesterA,E

ASchool of Molecular and Life Sciences, Curtin University, GPO Box U1987,

Perth, WA 6845, Australia.BLAQV-REQUIMTE, Chemistry Department, NOVA School of Science and Technology,

Universidade Nova de Lisboa, 2829-516 Caparica, Portugal.CPOLYMAT University of the Basque Country UPV/EHU,

20018 Donostia-San Sebastian, Spain.DIKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain.ECorresponding author. Email: d.silvester-dean@curtin.edu.au

In this mini review, we highlight some key work from the last 2 years where ionic polymers have been used as a catalyst toconvert CO2 into cyclic carbonates. Emerging ionic polymers reported for this catalytic application includematerials such as

poly(ionic liquid)s (PILs), ionic porous organic polymers (iPOPs) or ionic covalent organic frameworks (iCOFs) amongothers. All these organic materials share in common the ionic moiety cations such as imidazolium, pyridinium, viologen,ammonium, phosphonium, and guanidinium, and anions such as halides, [BF4]

–, [PF6]–, and [Tf2N]

–. The mechanistic

aspects and efficiency of the CO2 conversion reaction and the polymer design including functional groups and porosity arediscussed in detail. This review should provide valuable information for researchers to design new polymers for importantcatalysis applications.

Keywords: ionic liquids, polymers, CO2 fixation, cycloaddition reaction, heterogeneous catalysis, poly(ionic liquids),CO2 conversion, catalysts.

Received 31 July 2021, accepted 24 September 2021, published online 25 October 2021

Introduction

The growing atmospheric concentration of greenhouse gases (inparticular carbon dioxide, CO2) from industrial processes is a

matter of widespread public concern due to the resulting envi-ronmental problems associated with global warming and unpre-dictable climate change.[1–3] In order to meet CO2 emission

reduction targets, several strategies have been sought to captureCO2 directly from different sources, in particular carbon captureand storage (CCS).[4] Nevertheless, the processes associated to

this approach are highly energy intensive and less economicallyviable compared with CO2 capture and utilization (CCU),[5]

which focusses on the chemical transformation of CO2 and itsutilization as a carbon source for the production of fuels and

commodity chemicals.[6–8] Therefore, a viable strategy toremove CO2 from the atmosphere is to convert the gas viachemical processes to useful chemicals[9,10] or polymers.[11]

Using CO2 as an environmentally friendly and economicallyviable source of carbon can only be achieved by addressing thechallenges in the underlying principles of CO2 activation, design-

ing appropriate reactors and integration of processes. One of themost effective routes for CO2 fixation is cycloaddition of CO2

with epoxides to produce cyclic carbonates without by-products

(see Fig. 1), in a complete atom economy reaction that meets thecriteria of green chemistry.[12–14] Cyclic carbonates are ofparticular interest in applications such as aprotic polar solvents

and electrolytes in secondary batteries, precursors for polycar-bonate and polymeric materials synthesis, and intermediates inthe production of value-added chemicals such as dialkylcarbo-

nates, glycols, carbamates, pyrimidines, and purines.[13,15,16]

Theuse of a catalyst in this reaction is essential for reducing thetemperature and the pressure to make it efficient and economi-

cally viable. The development of new catalysts has been exten-sively investigated over the last few years, in particularhomogeneous catalysts such as organometallic complexes andmetal oxides, onium salts, ionic liquids, Schiff bases, andmono or

ReviewReview

*Debbie S. Silvester was awarded the 2021 Le Fevre Medal from the Australian Academy of Science.

OO

O OR

R

+ CO2

Catalyst

epoxide cyclic carbonate

Fig. 1. Synthesis of cyclic carbonate from CO2 and epoxide.

CSIRO PUBLISHING

Aust. J. Chem. 2021, 74, 767–777

https://doi.org/10.1071/CH21182

Journal compilation � CSIRO 2021 Open Access CC BY www.publish.csiro.au/journals/ajc

RESEARCH FRONT

binarymetal-free catalysts.[17] To bridge the gap between homo-

geneous and heterogeneous catalysts in the CO2–epoxide cou-pling reaction, organic polymers have emerged over the pastseveral years as efficient catalysts with unprecedented inherent

structural features.[16,18–20] The main feature of these polymersis that they are ionic in nature and include many ionic groupssuch as cations and anions similar to the ones used in ionic liquidchemistry (shown in Fig. 2). Owing to their design versatility

and polymeric materials, several families of polymers includingmaterials with high surface area such as porous organic poly-mers have been designed including ionic groups. The goal of this

mini review is to show some examples of the different types ofionic polymers that have been reported in the scientific literaturein the last 2 years for CO2 conversion.

Recent reviews have focussed on the design of and progressaround ionic polymers for the CO2 cycloaddition reac-tion,[14,16,18,21,22] with minor mentions of poly(ionic liquid)sor ionic polymers. To our knowledge, only one review focussed

only on imidazolium-functionalized ionic polymers as hetero-geneous catalysts, only partially addressing the subjectreviewed here.[18] To inspire the design of new ionic polymers,

it is timely to summarize and discuss recent strategies adoptedfor the CO2 conversion reaction that could be applied to othercatalytic reactions. In this mini review, we briefly overview the

numerous categories of ionic polymers, with a discussion on thecareful selection of cation and anion pairs, polymer backbone,and functional groups of newly emerging ionic polymers.

Only ionic functionalized organic materials are consideredfor this review while ionic liquid-supported inorganic materials(graphene oxide (GO), SiO2, CNTs, TiO2, etc.) and inorganic–organic hybrid materials (polyhedral oligomeric silsesquioxane

(POSS), metal organic frameworks (MOFs), zeolitic

imidazolate frameworks (ZIFs), etc.) are intentionally excluded.

First, an introduction to the mechanism of CO2 conversion andsome crucial insights into CO2 activation are provided, followedby a brief overview of the various sub-classes of ionic polymeric

materials used for CO2 conversion. Special emphasis is given tothe strategies for improving catalytic performance of thecharged polymers. The aim of the review is to provide someinsights into the possibilities of using emerging ionic polymeric

materials for the CO2 cycloaddition reaction.

Mechanism of CO2 Cycloaddition Reaction

CO2 is thermodynamically and kinetically stable, which limits itsutilization as a feedstock for organic synthesis. One way to tackle

this challenge is to choose products with a fairly high free energy,thus making the reaction with CO2 thermodynamically feasible.An example is the synthesis of ethylene carbonate from ethyleneoxide and CO2, which is a highly exothermic reaction with

DHrxn¼ –144 kJ mol�1 and has an almost 100 % atomic effi-ciency. The ring-opening of the three-membered epoxide releasesring strain energy, which is sufficient to drive the reaction.[23]

In principle, a cycloaddition reaction of CO2 with an epoxideto generate a cyclic carbonate often follows two pathways:(1) epoxide activation; or (2) CO2 activation; both pathways are

depicted in Fig. 3.[19,23] In the epoxide activation route, anucleophile attacks the less hindered carbon atom on the epox-ide, while in the CO2 activation route, the nucleophile attacks

the electrophilic carbon on CO2. For epoxide activation, thegenerally accepted mechanism using polymeric ionic liquids orionic polymer catalysts involves the following steps, as shownschematically in Fig. 4. Step (1): epoxide activation by the

interaction of Lewis or Brønsted acid sites on the poly(ionic

Rabia Jamil completed her M.Sc. at Quaid-i-Azam University, Islamabad, Pakistan. Her M.Phil. research was carried out in

the same university under the supervision of Dr Muhammad Shahid Ansari, focussed on the synthesis of nanomaterials with

various surfacemodifications for electrocatalysis andmaterials chemistry for renewable energy. Shewill soon begin her Ph.D.

studies under the supervision of A/Prof. Debbie Silvester (her start has been delayed due to travel restrictions following

COVID-19), focussing on the use of ionic liquids and poly(ionic liquids) for gas sensing and electrochemical applications.

Liliana C. Tome received a M.Sc. in Materials Derived from Renewable Resources (2008) from the University of Aveiro and a

Ph.D. in Engineering and Technology Sciences – Chemical Engineering (2014) from Instituto de Tecnologia Quımica e

Biologica Antonio Xavier (ITQB NOVA), Portugal. After her post-doc (2015–2018) at the same institute, she moved to

POLYMAT – University of the Basque Country (Spain) with a Marie Skłodowska-Curie Individual Fellowship. In 2020, she

joined LAQV-REQUIMTE, NOVA School of Science and Technology | FCT NOVA (Portugal), where she is an Assistant

Researcher and recently received the L’Oreal PortugalMedal of Honor forWomen in Science 2020. Her areas of research are

centered on iongels and poly(ionic liquid)-based materials for gas separation and bioelectronics.

David Mecerreyes is an Ikerbasque Research Professor at POLYMAT-University of the Basque Country (Spain). He received

his Ph.D. from University of Liege in 1998. After a post-doc (1998–2000) at IBM Almaden Research Center (California), he

moved to Spain to work at CIDETEC. In 2010, he moved to POLYMAT where he became the leader of the Innovative Polymers

Group. His area of expertise is polymer synthesis of ionic and electroactive polymers, such as poly(ionic liquid)s, polymer

electrolytes, redox polymers, and PEDOT type conductive polymers. His current research interests involve the application of

innovative polymers in emerging technologies such as organic batteries and bioelectronic devices.

Debbie Silvester is an Associate Professor and Australian Research Council (ARC) Future Fellow at the School of Molecular

and Life Sciences at Curtin University in Perth, Australia. She completed her M.Sci. in Chemistry at the University of Bristol

and her Ph.D. at the University of Oxford, UK. Her research interests include electrochemistry in ionic liquids,

electrochemical reaction mechanisms, gas sensing, explosives detection, electrodeposition, and formation of modified

electrodes for sensing applications. She has been awarded several national and international awards for research, including

the 2021 Le Fevre medal (Australian Academy of Science) and the 2021 Analyst Emerging Investigator Lectureship (Royal

Society of Chemistry).

768 R. Jamil et al.

liquid) (PIL) cations with the weakly basic oxygen atom on the

epoxide. In metal-containing catalysts (Lewis acid site), theinteraction is via a metal–oxygen coordinative bond, whereashydoxyl-, amine- or carboxyl-containing ionic polymers

(Brønsted acid site) interact with epoxide through the hydrogen

bond. Step (2): ring-opening of the epoxide initiated by nucleo-philic attack of the halide anion (X–) on the least hinderedcarbon atom of the epoxide, leading to the formation of an

alkoxide intermediate. The ring-opened epoxide is stabilized bythe counter cation present in the polymeric framework. Step (3):CO2 insertion requires activation of CO2 by nucleophilic or

electrophilic attack. CO2 is a linear, apolar molecule with twopolar (C=O) bonds, with electropositive and electronegativecharges on the carbon and oxygen atoms, respectively. The

electronegative oxygen acts as a nucleophile and activates CO2

by attacking the electrophilic carbon atom to form a carbonateintermediate. Step (4): the open-chained carbonate undergoesintramolecular cyclic elimination. Step (5): finally, displace-

ment of X– leads to the formation of a five-membered cycliccarbonate, together with regeneration of the catalyst. The choiceof the PIL or ionic polymer cation, and the use of a halide anion

(as the nucleophile) are crucial for the generation of five-membered cyclic carbonates.[21] The nucleophile should havea good balance of nucleophilicity – to facilitate the ring-opening

process – as well as the ability to act as a leaving group so that itcan be easily displaced to ensure cyclic carbonate formation.Similarly, to insert CO2 into the molecule, the PIL or ionicpolymer cation needs to stabilize the alkoxide intermediate

through favourable chemical interactions.[23]

The CO2 activation pathway proceeds when a nucleophilesuch as anN-heterocyclic carbene (NHC), e.g. 1,5,7-triazabicyclo

[4.4.0]dec-5-ene or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU),

N

N

N NHX–

Halide(X = Br, Cl)

NN

RR

+ +

+

(b)(a)

FF

–

–

F

F3C CF3

NH2

H2N

N

–O

O O

–R O

–N NS S

O

O

O

O

FF

FF

F

F

F

B P

Tetrafluoroborate Hexafluorophosphate

++

+

+

RR

P N

N

N

Imidazolium Pyridinium Guanidinium

Viologen

Quaternaryphosphonium

Quaternaryammonium

Glycinate Amino acid

Bis(trifluoromethylsulfonyl)imide Imidazole anion

Fig. 2. Chemical structures of (a) common cationic moieties; and (b) anionic moieties in ionic polymers.

Nu–

Nu–

O–

O–

O

O

O

O

OO

R

O

OC Nu

Nu

CO2

M or HBD M or HBD

CO2 activation pathway Epoxide activation pathway

M = metal centre HBD = hydrogen bond donor groupNu = nucleophilic group

Fig. 3. General mechanistic pathways for the cycloaddition of CO2 with epoxide to form a cyclic carbonate.

O

O

O

O

O

O

O

O

O

O

O

O

CO2

R

C

R

R

R

R

R

cycliccarbonate

Step 5Step 1

Step 4

Step 3

Step 2

–

–

–

HBD

HBD

HBD

HBD

X

X

X

HBD–X(or M–X)

epoxide

X–

Fig. 4. General mechanism of cyclic carbonate synthesis using ionic

polymers, adapted from data in refs [21] and [23].

Emerging Ionic Polymers for CO2 Conversion 769

attacks the electrophilic carbon of CO2, as depicted in Fig. 5.

Recent studies show that PILs/ionic polymers can act as pre-catalysts for polyNHCs.[21] A possible CO2 activation pathwayusing NHCs is as follows: (i) nucleophilic attack of CO2 by an

NHC catalyst to form a zwitterionic NHC carboxylate adduct;(ii) the zwitterionic adduct attacks the epoxide to produce a newzwitterion; (iii) then, the alkoxy anion nucleophile formed

attacks the electropositive carbonyl carbon atom to produce acyclic carbonate by intramolecular cyclic elimination.[21]

In the next sections, different emerging ionic polymers are

discussed from the standpoint of ease of introduction of thedesired functional group, and the atomic-level control over theirstructures and compositions. It is worth noting that the classifi-cation of these emerging ionic polymers is not easy, andwe have

kept the nomenclature and names proposed by the authors. Ingeneral, most of the emerging ionic polymers have in commonthe ionic functionalities (anions and cations), the inclusion of

other functionalities, and their use as heterogeneous catalyststhat can be recovered and reused several times, as well as thedifferent types of porosity.

Functionalized Ionic Polymers for CO2 Conversion

Ionic polymers contain abundant ionic sites that retain someof the

molecular ionic compounds, such as ionic liquids, making themattractive as heterogeneous catalysts for CO2 conversion. Dif-ferent approaches have been adopted to modulate the type and

density of ionic sites, as well as the porosity of ionic polymers, toupgrade their catalytic performances.[18,26–28] One efficient wayis to utilize the unique advantages of ionic hyper-crosslinked

polymers due to their large surface area, versatile architectures,and diverse ionic building blocks.[29,30] Wang et al. synthesizedimidazolium-functionalized hyper-crosslinked polymers

(HCPs) using a series of benzylimidazole salts as the (charged)ionic monomers and a,a0-dibromo-p-xylene as a (neutral) cross-linking agent.[31] The connection between the phenyl ring andthe imidazolium cation facilitated a hyper-crosslinking reaction

through FeCl3-catalyzed Friedel–Crafts alkylation. The fasci-nating structural design with large surface area (up to 1017m2

g�1) showed a remarkable CO2 uptake (up to 3.05mmol g�1 at

273K), as well as better CO2 adsorptive selectivity over N2. In

addition, ionic HCPs catalyzed the epoxide reaction usingsimulated flue gas (15% CO2 in N2 v/v) with high yield in theabsence of solvents and additives.[31]

Several strategies have been developed for tethering hydrogenbond donor (HBD) groups onto ionic moieties of ionic poly-mers.[18,32,33] HBD groups strengthen the H-bonding interactionbetween the catalyst and the substrate and stabilize the reaction

intermediates. The impact of surface groups on the catalyticactivity of imidazolium-based ionic polymers follows the orderhydroxyl. alkyl. amino. carboxyl. sulfite group.[29] The

better performance of the hydroxyl-tethered ionic polymer isattributed to the formation of medium-strength hydrogen bonds,which is an important pre-requisite for epoxide activation

and ring opening.[29] By increasing the number of adjacenthydrogen-bond donors, the catalytic activity of the ionic poly-mer can be increased. For instance, a high density of urea groupsin the ionic polymer backbone serves as dual hydrogen bond

donors for the activation of epoxide C�O bonds. Urea-functionalized imidazolium-based ionic polymers exhibited a2-fold higher catalytic activity compared with their counterparts

without urea groups.[34]

Poly(Ionic Liquid)s (PILs)

The reactivity of PILs for CO2 fixation or conversion can bealtered bymanipulating the composition of the PIL or varying the

chemical structures of the ions. A new family of PILs based onaromatic crosslinked poly(azomethinepyridinium) salts contain-ing Cl–, [TFSI]– (TFSI¼ bis(trifluoromethyl)sulfonylimide),

and [PF6]– anions were prepared in a one-step synthesis from

diamine methylpyridinium salts and isophthaldehyde.[35] Thedifferent anions were found to influence the thermal stability,morphology, and catalytic efficiency of the resulting poly

(azomethinepyridinium) PIL. PILs containing the chlorideanion were the most promising catalysts, with 98 % substrateconversion from 0.5 mol-% of catalyst at 1008C and 3 bar

(300 kPa) of CO2.[35]

Amino acid ionic liquids are acid–base multifunctionalmaterials that possess a carboxylic acid functionality, an amino

group, and nucleophile in a singlemolecule. Zhou et al. preparedamino acid paired PILs (AA-MPILs) as highly active metal- andhalogen-free heterogeneous organocatalysts (see Fig. 6).[36] It

was found that manipulating the amino groups on the cation wascrucial to increase catalytic efficiency. The amino acid groupsexhibit an ‘enhancement effect’ even stronger than that ofhydroxyl or halogen anions, due to the better leaving ability of

the carboxyl group. Among the various amino acid groupsexamined, those with dual carboxyl groups exhibited betterefficiencies.[36] Apart from this study, investigations on PILs

containing amino acid ionic liquids as ionic moieties for CO2

conversion are extremely scarce.

Functionalized PILs

Introducing Brønsted acid sites and Lewis basic sites incatalysts is helpful for promoting smooth cycloaddition reac-tions by activating and accelerating the ring-opening process of

epoxides.[37] Therefore, designing PILs with high-densitynucleophilic and electrophilic sites is a promising approach toattain high catalytic activity.[25,37] For this reason, functionali-

zation of PILs by introducing HBD groups (e.g. hydroxyl,amine, carboxyl), or using a halogen as the anion are usefulstrategies. The synergistic effect of an HBD and a nucleophile

O

O

O O

O

OC

O

O

O

O

O

O– O–

O–

R

R

R

R

cycliccarbonate

Step 4

A = anion= cation containingpolymer/ionic polymer

Step 1

Step 3 Step 2

AA

A

A–

epoxide

+

+

+

+

+

Fig. 5. General mechanism of cyclic carbonate synthesis via the CO2

activation pathway, adapted from data in refs [24] and [25].

770 R. Jamil et al.

can lower the activation energy for the rate-determining step forCO2 fixation into cyclic carbonates.[9,38]

In order to investigate the effects of different groups on the

catalytic activity of PILs,Ying et al. used2-(dimethylamino)ethyl

methacrylate (DMAEMA) as a precursor to tailor different

ionic liquids (ILs) and their corresponding PILs by self-

polymerization.[39] The activity of the PILs decreased in the

order: P(DMAEMA-EtOH)Br. P(DMAEMA-PrOH)Br. P(DMAEMA-Pr)Br. P(DMAEMA-Bu)Br. P(DMAEMA-Et)Br. P(DMAEMA-Pr)SO3. The basicity of the Br– anion sig-

nificantly boosted the activity compared with the SO3– anion.

Hydroxyl-rich PILs showed equivalent activity to their bulk ILs,but superior activity to other PILs due to the synergistic effect ofthe hydroxyl anion and the halogen nucleophile.[39] Another

study showed that the yield of propylene carbonate (PC) can beincreased from 72% to 94% (at 1058C, 2 MPa CO2, 3 h) byfunctionalizing the PIL with an HBD, giving a yield comparable

with that using bulk ILs (95%).[37] Both hydroxyl groups of theILs and HBDs can form strong H-bonds with propylene oxide(PO) and help to initiate the ring-opening process. The functio-

nalized PILs demonstrated good conversion and selectivity(.98%) for various epoxides to form their corresponding cycliccarbonates.[37]

Li et al. synthesized heterogeneous PILs with thiourea (as an

electron-deficient centre) and ammonium bromide (as a nucleo-

philic site).[40] The two hydrogen atoms on thiourea form strongH-bonds with epoxide; thus they can activate and initiate the

reaction in synergy with the bromide ion.[40] Zhang and co-workers covalently integrated porous poly(ionic liquid)s(PPILs) with covalent organic frameworks (COFs) to form a

core–shell hybrid (denoted PPIL@COF) for the first time.[41]

The hydroxyl groups in the interfacial layer of the PPIL@COFact as Brønsted acid sites that could cooperate with bromide for

the activation of epoxides.[41] Zhang et al. combined bothhomogeneous and heterogeneous catalysts, which resulted inhigh catalytic activity and easy separation of catalysts from the

reaction products.[42] A proof-of-concept study used an amine-functionalized imidazolium PIL monolayer attached on meso-porous silica particles to ensure easy separation of catalysts fromthe reaction system, while swelling of the surface-attached PIL

chains allowed a local homogeneous catalytic reaction to occur.Lewis acid–base interactions with the amine, together with thereactive carbon at the C(2)-position of the imidazole ring could

activate CO2 for the cycloaddition reaction. The counter ions ofthe PILs were found to significantly influence the efficiency ofthe catalyst, in the order: Br–. [PF6]

–.CH3COO–, owing to

differences in electrophilicity and steric hindrance.[42]

Apart from HBDs, metal sites can also act as electrophiles,which together with the halogen anion greatly enhance reactionkinetics owing to the dual activation mode.[43] As a proof of

concept, Xie et al. synthesized a PIL containing an electrophileand a nucleophile by free-radical polymerization ofimidazolium-basedmonomers and bipyridine (Py)-basedmono-

mers, followed by efficient post-synthetic metallization. Cycliccarbonate yields of .92 % (1 atm (100 kPa) and 308C for 18–48 h) were obtained for a broad range of epoxides owing to thevery high density of bromide anions and zinc sites (six bromide

anions in a monomer, 5.15 wt-% loading of Zn).[44] In anotherstudy, PIL-supported NHC silver complexes (poly(NHC-Ag/IL)) containing active sites of ILs andNHC-Ag complexes in the

network were able to catalyze the cycloaddition reaction undermild conditions without co-catalyst.[45] A proposed mechanismfor the reaction of epoxide with CO2, catalyzed by poly(NHC-

Ag/IL) is a dual activation of the epoxide ring with a Lewis acidsite (NHC-Ag) together with a nucleophilic reagent (Br–),resulting in ring-opening of the epoxide, followed by the

addition of CO2. The poly(NHC-Ag/IL) exhibited a higher yield(83%) compared with the corresponding PIL (74%).[45]

Hypercrosslinked PILs

Until now, great efforts have been dedicated to modulate the

chemical composition and porosity of PILs for CO2 absorptionand conversion.[21] Hierachical meso and macroporous PILshave drawn tremendous attention because of their high surface

area. The density of nano pores as well as the pore diameters areessential to promote rapid mass transfer during CO2 capture andconversion.[43] The synthetic strategy should be carefully tuned

to take full advantage of so-called ionic HCPs. The predominantmicroporosity and high surface area of HCPs is advantageousfor CO2 capture, and ILs or functionalized-IL monomers pro-

vide the catalytic sites for CO2 conversion.[46] A Friedel–Crafts

alkylation reaction involving an electrophilic substitution step iscommonly used to prepare HCPs. The choice of the cation in ILmonomers can affect the linking between the cross-linkers and

IL monomers.[29,46] In order to regulate the pore structures,surface functional groups, and density of ionic moieties, Sang

N

N

NH2

[AVIM]Br [PAD]Br

+ +

[AA] = H

HN

O

O O

O–

–O

O O

O O

O O

OO

O–O–

O–

HH

[IDA] [Glu] [Asp] [Ala]

NH2 NH2NH2

NH2NH2NH2

N+N+N+OH–Br–Br–

OH– exchangeNN N

H2N

[Gly]

[AA]

[PAD][AA][PAD][OH]

**

**

*

**

**

*

**

Amino acid

neutralization

80°C

reflux

Fig. 6. Preparation of AA-MPILs. Reprinted with permission from Zhuo et al.[36] Copyright 2019 American Chemical Society.

Emerging Ionic Polymers for CO2 Conversion 771

and Huang prepared benzimidazole-based hyper-crosslinked

PILs (denoted HPILs). In the FeCl3-catalyzed Friedel–Craftsalkylation reaction, the quaternization reaction between benz-imidazole and benzyl halides produced imidazolium salts for

HPILs. The amount of benzimidazole was used to control the Ncontent and the porosity of the resultantmaterials (see Fig. 7).[46]

In another study, Gai and co-workers demonstrated theimportance of using an appropriate number of benzene rings

in IL monomers to promote the Friedel�Crafts alkylation toform HPILs. It was shown that using a small number of benzenerings impedes Friedel�Crafts alkylation owing to the limited

number of active sites, while a large number of benzene rings inthe structure creates steric hindrance, preventing the formationof an HCP.[47] The resultant porous benzene-rich PIL had a good

CO2 absorption strength, high selectivity for CO2 over N2, andgood subsequent CO2 conversion to cyclic compounds.

The majority of porous PILs have amorphous structures,meaning they do not contain ordered porosity spacing. Wang

et al. proposed the concept of an ordered porous crystalline PILas a way to improve both the adsorption capacity of CO2 as wellas its conversion.[48] Structure-controlled, ordered crystalline

porous PILs were synthesized via a thermodynamically con-trolled Schiff base condensation from IL-derived ionic salts andneutral monomers. In the case of an ordered porous material, the

chemical composition and pore structure play a vital role indetermining the surface affinity for CO2, while the ionic moie-ties act as active catalytic sites for CO2 conversion.

Ionic Porous Organic Polymers (iPOPs) or Porous IonicPolymers (PIPs)

Porous organic polymers (POPs) are new and highly versatileorganic polymers with high surface areas, tuneable pore sizes,and tuneable catalytic active sites.[18,49] POPs include HCPs,

porous aromatic frameworks (PAFs), porous polymer networks(PPNs), conjugated microporous polymers (CMPs), COFs, andcovalent triazine frameworks (CTFs).[14] Furthermore, the

functionalization of POPs with nitrogen-containing groups (e.g.amines, imines, triazines, carbazoles, imidazoles, and azolinkages) has been shown to increase CO2 capture and conver-

sion abilities.[14] Porous ionic polymers (PIPs) are a class ofPOPs that have controllable porous structures, abundant ionicsites, good ion exchange properties, and tuneable chemical

functionalities.[50] In particular, halogen-based PIPs have betterCO2 capture ability due to the strong dipole–quadrupole

interactions of their ionic moieties with CO2. As discussed

previously, the halogen anion plays the role of an activenucleophile for the CO2 cycloaddition reaction.[50] Moreover,IL moieties in the polymer backbone have an intrinsic hydro-

philicity that can chemically absorb nearby water molecules viaH-bonding. A sufficient amount of water can enhance the cou-pling of CO2 with epoxide by forming H-bonds with the oxygenatom on the epoxide.[37,50]

Viologen-Based iPOPs

Dicationic quaternary salts of 4,40-bipyridine are referred toas the viologen family. Viologen-based ionic polymers exhibit

unique properties because of their active ionic sites and adjust-able redox states, rendering them useful for diverse applica-tions.[50] Different synthetic methods have been adopted,

including ionothermal trimerization, C–C coupling reactions(e.g. Sonogashira coupling reaction and Heck reaction), theZincke reaction, imine condensation, and the Menshutkin reac-

tion. Among them, the Menshutkin reaction is a convenient,highly efficient method that represents a catalyst-free route toachieving viologen-based iPOPs.[50] Zhang et al. synthesized atask-specific crystalline polymer enriched with Br– anions

paired with viologen ionic sites to catalyze CO2 into cycliccarbonates and highlighted the role of chemically absorbedwater molecules, which are often observed in ILs and IL-

based solid materials.[50] Chemically absorbed water moleculescan be regarded as HBD co-catalysts that work together with Br–

anions in the activation and ring opening of epoxide (see

Fig. 8).[50] A comparison between Br– and Cl– anions asnucleophiles revealed that viologen paired with a Br– anionshowed better activity owing to its higher nucleophilicity andbetter leaving ability than the Cl– anion.[50] In another study,

viologen-based iPOPs containing dual active sites i.e. Si�OHgroups (HBD) and the Br– anion, demonstrated CO2 fixationunder ambient pressure and low temperature without using co-

catalysts or solvents.[51]

Metallosalen-Based iPOPs

Metallosalen-based PIPs combine the merits of both homo-

geneous organometallics and heterogeneous porous ionic cata-lysts for the CO2 cycloaddition reaction.[30] In contrast totraditional metallosalen catalysts, incorporating metallosalen

complexes into a porous framework containing free mobileanions can provide a co-operative environment to promote

X

X

HN N

X = Cl, Br

+DCE 80°C

HN

HN

N

N

X

X

FeCl3

Ionic I

Ionic II

Self-condensation

Quaternization

+

+

–

–

Fig. 7. Schematic procedure for the synthesis of the benzimidazole-based hyper-crosslinked poly(ionic

liquid)s. DCE¼ dichloroethane. Reprinted with permission from Sang et al.[46] Copyright Elsevier (2020).

772 R. Jamil et al.

catalytic CO2 conversion.[52] The nature of the metal, the spatial

distance between the metal active site and the halogen anion,

and the ratio of ionic:metal units in metallosalen-based iPOPsplay a crucial role in the catalytic activity enhancement towardsCO2 cycloaddition.[30] In order to study the cooperative acti-

vation from the metal (Lewis acid) and halogen anions(nucleophile), Li et al. prepared a series of porous metallosalen(M¼Co, Al, Zn) hyper-crosslinked ionic polymers byFriedel�Crafts alkylation and quaternization.[30] Among them,

the cobalt salen-based catalyst showed the highest activity.Theoretical investigations revealed that the reduced energybarrier of the ring-opening process associated with Co is due

to its efficient coordination with the oxygen atoms of epoxidecompared with Al and Zn.[30] After the polarization of the C–Obond by cobalt ions, bromide anions attack the less-hindered

side of the coordinated C on the epoxide to form a Co-coordinated bromide alkoxide intermediate, which then attacksCO2 to form alkylcarbonate anions. Lastly, an intramolecularring-closing reaction leads to the formation of cyclic carbonates

(see Fig. 9).[30] Valence state and corresponding coordinationgroups can influence the rate-determining step of the cycload-dition reaction and nucleophilicity of the halogen anions.

Density functional theory results showed that Co-salen withCoII exhibited the lowest energy barrier of the reactions, as wellas an enhanced nucleophilicity of the Br– anion.[52]

Metalloporphyrin-Based iPOPs

Porphyrins and their derivatives are p-conjugated macro-cycles rich in nitrogen atoms that facilitate the coordination of

metal ions.[17] When integrated into functional polymeric mate-rials, metalloporphyrins provide electron-deficient Lewis acidcentres for epoxide activation and ring opening.[17] Using an‘all-in-one’ strategy, Luo et al. integrated imidazole, a halogen

anion, and a metal centre into a single catalyst and prepared a

heterogeneous zinc porphyrin-based cationic polymer (ZnPor-CP)for CO2 fixation.

[17] The abundant imidazolium functionalitiesin the covalently linked skeletons and the spatially separated

active Zn-porphyrin sites co-operatively enhanced the reactionunder mild conditions with good recyclability.[17] In anotherstudy, the same authors demonstrated the use of a click-based

O

O

CO2

OO

Br

Br–

O

O

O

O

O–

–

R

R

R

R

R

HN

N+

Br–

Br–

Br–

Br–

N+

Br–

Br

N+

+N

+N

+N

N+

H

xH2O

H

H HHO

O

O

H

HH

H HHO

O

O

H

HH

H HHO

O

O

*

* *

*

**

**

VIP-Br

Fig. 8. A proposed catalytic reaction mechanism for the cycloaddition of CO2 with epoxides over the catalyst viologen-based

porous ionic polymer (VIP-Br) with Br– anions andH-bondedwater molecules. Reproduced fromZhang et al.[50] with permission

from the Royal Society of Chemistry.

N N

OO

N

N

N

+N

N+

N

OO

OO

O

O

OO

CO2

RR

R

R

O

O

O

R

X = CoOAc, AlCl, Zn

Cooperativeanalysis

Fig. 9. Metallosalen hyper-crosslinked ionic polymer (M-HIP) structure

and tentative catalytic mechanism for the CO2 cycloaddition of epoxides by

Co-HIP. Reprinted with permission from Li et al.[30] Copyright 2020

American Chemical Society.

Emerging Ionic Polymers for CO2 Conversion 773

reaction for the synthesis of a Co porphyrin-based ionic polymer

with abundant acidic metalloporphyrin and imidazolium ILsites. The activity of the Co porphyrin ionic polymer decreasedwith the introduction of halogen anions in the order Br–.Cl–

. I–. It was suggested that the Br– anion provides the optimumbalance of nucleophilicity and leaving ability compared with theCl– and I– anions.[53]

Porous Ionic Copolymers

Typical flue gases contain ,15% CO2 along with large

amounts of N2. The direct conversion of diluted CO2 is chal-lenging, because the catalyst needs to be highly active andselective towards relatively small concentrations of CO2 in the

presence of N2. Hui et al. synthesized an ionic co-polymer bycombining protic ILs with porous polymers for efficient con-version of diluted ‘waste’ CO2 (1.0 bar (100 kPa), 15%CO2 and85%N2) into cyclic carbonates under ambient conditions.[54] In

another study, Sun et al. co-polymerized bi-functionalized ILscontaining ether and amine groups with divinylbenzene (DVB)to produce halogen-free ionic porous copolymers.[24] The max-

imum Brunauer–Emmett–Teller (BET) specific surface area ofthe copolymer was 508 m2 g�1 with a pore volume of 0.90 cm3

g�1. The construction of porous materials and the addition of

functional groups amplified both CO2 capture capacity andcatalytic performance of the IL polymer for CO2 conversion.

[24]

Ionic Porous Organic Frameworks

Porous organic frameworks are a subclass of POPs with acertain degree of ordered networks. Among them, N-rich ionic

porous organic frameworks (POFs) contain exchangeablecounter ions that act as nucleophiles for the epoxide ring-opening process. Recently, Hussain and co-workers employed

metal-loaded triaminoguanidinium-based cationic frameworksfor the catalytic conversion of CO2.

[55] The guanidinium corenot only provides suitable anchoring sites for Zn ions, but also

compensates for the poor physiochemical instability of iPOFs.ZnII (47.2%) acts as a Lewis acid to augment the catalyticactivity related with such frameworks.[55]

Ionic Covalent Organic Frameworks (iCOFs)

COFs are crystalline, porous materials with two-dimensional(2D) or three-dimensional (3D) structures. They are synthesizedfrom organic building blocks using reversible covalent bonds.

Significant features include high stability, accessible openchannels or nanopores with uniform sizes (ranging from ang-stroms to nanometres), and extremely low density.[56,57]

Although COFs have been used as heterogeneous catalysts forthe CO2 cycloaddition reaction, the low catalytic efficiency and/or use of co-catalysts limits their application. One way to

overcome this is to functionalize a COF with an imidazoliumsalt, which provides cationic and anionic sites in the nanoscopicchannels as active centres towards CO2 conversion. Qiu et al.demonstrated that an imidazolium salt-functionalized COF

exhibited ,50 times better turnover efficiency compared withthe transition metal-free COF catalyst.[56] In another study, ILsgrafted onto the channel walls of COFs exhibited good catalytic

activity for the synthesis of cyclic carbonates from epoxides andCO2 in the absence of solvents and co-catalysts.[57]

Zhong et al. upgraded the catalytic ability of COFs by

devising a ‘host–guest’ catalytic system via integrating anitrogen-rich COF and an imidazolium-based ionic polymer.The unification of two catalytically active components not only

facilitated the epoxide and CO2 cycloaddition reaction, but also

gave high absorption selectivity of CO2 over N2.[58] Hence, the

entrapment ability of COFs makes them a good host to integratewith ILs or ionic polymers.

Ionic Covalent Organic Polymers (iCOPs)

Covalent organic polymers (COPs) are composed of well-defined periodic frameworks of covalently bonded organicgroups. Functional building units control the catalytic propertiesof COPs, as well as the environment around catalytically active

sites.[59] A wide flexibility in monomer selection and tailoredsurface manipulation with polar groups allow COPs to be usedfor the cycloaddition reaction of CO2. For example, Kim et al.

reported a nanoporous hydrocarbon framework with graftedquaternary ammonium salts for the CO2 cycloaddition reac-tion.[60] The steric hinderance of the alkyl units decreased the

catalytic activity in the order CH3NMe3þ.CH3NEt3

þ.CH3NiPr2Et

þ. Thus, a higher loading of less sterically hinderedammonium salt led to higher CO2 absorption and subsequent

faster cycloaddition reaction kinetics.[60] A cationic COP basedon an aluminium salen (Al salen) ligand (Lewis acid site) syn-thesized via a post-synthetic metalation strategy exhibited goodcatalytic efficiency. Furthermore, COP-Al retained its effi-

ciency for up to five reaction cycles.[61] Recently, Ayhan et al.reported efficient catalytic activity of viologen-porphyrin-basediCOPs due to the presence of multiple CO2-philic sites and

efficient non-covalent interactions with CO2.[62]

Charged Covalent Triazine Frameworks (cCTFs)

CTFs are nitrogen-rich porous structures with ultra-strong aro-

matic C=N linkages. CTFs are versatile materials for hetero-geneous catalysis owing to their high specific surface area withadjustable pore size, designable surface-active sites, and

exceptional physicochemical stability.[19] In comparison withtypical CTFs, charged CTFs are co-integrated with ionic moi-eties containing both electrophilic and nucleophilic centres

within the framework, eliminating the need for a co-catalyst.Coskun and co-workers first prepared a charged CTF via aconventional ZnCl2-catalyzed trimerization reaction using 1,10-bis(4-cyanophenyl)-[4,40-bipyridine]-1,10-diium dichloride as a

feedstock under ionothermal reaction conditions.[63] Themicroporous ionic CTF exhibited high specific surface area(1247m2 g�1), high CO2 uptake capacity (3.0mmol g�1 at

273K), and high yield (up to 99%) for propylene oxide under1.0 MPa of CO2 at 908C for 12 h. Cao and co-workers demon-strated the effects of polymerization temperature and the

monomer/ZnCl2 ratio on porosity and ionic surface propertiesof CTFs.[64] Although higher polymerization temperaturesincreased the mesoporosity, the decomposition of imidazolium

moieties and loss of spatially confined nucleophilic chlorideanions dramatically reduced the catalytic performance ofimidazolium-functionalized cationic CTFs synthesized at ele-vated temperatures.[64] The presence of abundant imidazolium

and triazine units is beneficial for excellent CO2 uptake capac-ity, but the synergistic effect between ionic moieties is crucialfor efficient catalytic activity. Considering the fact that both the

metal active sites and halogen anions can significantly enhancethe CO2 cycloaddition reaction under ambient conditions, thesame group integrated cobalt porphyrin into imidazolium-

functionalized cationic CTFs.[65] These mixed monomers ledto stable CTFs with multiple catalytically active sites that notonly increased CO2 absorption capacity, but also subsequent

774 R. Jamil et al.

CO2 conversion.[65] It is noteworthy that the choice of an

appropriate polymerization temperature is important to obtainabundant hierarchical porous structures without losing the highdensity of active sites.

Recyclability and Stability of Ionic Polymers

In addition to high efficiency, recyclability and stability are alsoimportant factors for catalysts before they can be used in prac-tical applications. IL-supported inorganic–organic hybrid

materials offer multiple conversions of CO2; however, the maindrawback of this type of hybrid material is its coordination bondbecause leaching of IL from the catalyst during the reaction can

occur, decreasing both the reactivity and stability.[18] Ionicpolymers with both a high activity and easy separation from theproduct have therefore been the focus of recent research. Owing

to the rich covalent chemistry and cross-linked network, ionicpolymers offer sufficient stability for CO2-based liquid-phasereactions. Table 1 demonstrates the excellent recyclability and

stability of ionic polymers; the majority can be reused at leastfive times without obvious loss of catalytic activity and selec-tivity. The choice of an efficient synthesis protocol and theintroduction of targeted active sites (such as HBD groups, metal

sites, nucleophiles) with high density and good dispersion canfurther enhance the stability of ionic polymers.[44] For example,a Co porphyrin-based ionic polymer synthesized via a click-

based reaction was recovered and reused eight times withoutchange in its composition.[53] The internal structure of the cat-alyst was unchanged and its activity remained above 98% after

eight runs. Therefore, abundant metal and IL sites and the

presence of stable covalent bonds in the structure can not only

improve the catalytic activity but also its reusability.

Conclusions and Future Prospects

Table 1 summarizes the reactions discussed in this mini review,including different epoxide substrates, emerging ionic poly-mers, and reaction conditions. The variety of substrates and

reaction conditions make it difficult to establish a comparisonand to benchmark among the different types of emerging ionicpolymers. In any case, integrating ionic moieties in organic

functional polymers provides tremendous opportunities toefficiently catalyze the cycloaddition of CO2 under ambientconditions owing to the strong van der Waals forces and elec-

trostatic interactions between ILs and CO2. Cations such asimidazolium, pyridinium, viologen, ammonium, phosphonium,and guanidinium, and anions such as halides, [BF4]

–, [PF6]–, and

[Tf2N]– have been thoroughly explored for the CO2 cycloaddi-

tion reaction. Moreover, the task-specific design of ionic poly-mers and including porosity in their structure allow easy controlover the catalytically active sites and cooperative interactions of

the functional groups, giving rise to highly selective CO2

adsorption and simultaneous conversion. It is likely that theperfect combination of a suitable design strategy and synthetic

protocol will open the doors for ionic polymers to be used inindustrial applications. But there are still several future chal-lenges and areas for research that we suggest to further develop

these materials, including:

� developing innovative ionic polymers with a high surface

area (SBET), tuneable porosity, and high density of ionic

Table 1. Summary of the epoxide used for CO2 conversion, catalyst and reaction conditions used, yield of product, reusability (number of runs), and

literature reference

Epoxide Catalyst Reaction conditions Yield [%] No. of runs Ref.

Epichlorohydrin [PDVB-HAVIM-C18]Br 808C, 0.1 MPa (CO2), 16 h 96.4 6 [38]

Epichlorohydrin VIP-Br 408C, 0.1 MPa (CO2), 72 h 99 5 [26]

Epichlorohydrin IHCP-OH(1) 1208C, 1 MPa (CO2), 2 h 90 5 [29]

Styrene oxide Co-HIP 808C, 0.1 MPa (CO2), 20 h 96 5 [30]

Epichlorohydrin IC2HCP-5b 1208C, 3 MPa (CO2), 4 h 81 5 [31]

Epichlorohydrin UIIP 1108C, 1 MPa (CO2), 1 h 99 6 [34]

Epichlorohydrin PAM-PyMe(Cl) 1008C, 0.3 MPa (CO2), 6 h, 0.5mol-% 98 4 [35]

1-Ethyl-2-phenylaziridine [PAD][IDA] 658C, 1 MPa (CO2), 12 h, 3.4mol-% 95 5 [36]

Propylene oxide HPIL-7 1058C, 2 MPa (CO2), 3 h, 1.5mol-% 94 5 [37]

Propylene oxide P(DMAEMA-EtOH)Br 1108C, 2 MPa (CO2), 3 h 98.5 5 [39]

Propylene oxide P-Cm3Lm1Nm2 1008C, 1.2 MPa (CO2), 12 h, 4mol-% 99 5 [40]

2-Methyloxirane PPIL@COFA-40 1008C, 2 MPa (CO2), 24 h 96 5 [41]

Epichlorohydrin SBA16-PIL-Br 1208C, 1.5 MPa (CO2), 4 h 90 – [42]

Epichlorohydrin P2D-4BrBQA-Zn 508C, 0.1 MPa (CO2), 24 h, 0.5mol-% 96 5 [43]

Propylene oxide Py-Im-6-Zn-5-SCD 1208C, 0.5 MPa (CO2), 1 h 94.5 6 [44]

Epichlorohydrin Poly(NHC-Ag/IL)s 1008C, 1 MPa (CO2), 3 h, 1 mol-% 87 5 [45]

Propylene oxide HPILs-Cl�2 708C, 0.1 MPa (CO2), 9 h 99 4 [46]

Epichlorohydrin HP-[BZPhIm]Cl�DCX-1 808C, 0.1 MPa (CO2), 24 h 98.9 5 [47]

Epichlorohydrin V-iPHP-1 608C, 0.1 MPa (CO2), 72 h 99 5 [51]

Epichlorohydrin ZnPor-CP 1208C, 1 MPa (CO2), 12 h, 0.08mol-% .99 – [17]

Propylene oxide CoTPP-PiP(Br) 1008C, 1 MPa (CO2), 10 h, 0.2mol-% .99 8 [53]

Styrene oxide PDVB-[AlTMG]Br-0.2 1008C, 0.1 MPa (CO2), 6 h 99 5 [54]

Epichlorohydrin HP(0.6DVB-[VMOEim][Gly]) 1308C, 0.1 MPa (CO2), 10 h 94 5 [24]

Epichlorohydrin ImIP@TT-COF 1208C, 0.1 MPa (CO2), 10 h .99 6 [58]

Epichlorohydrin NUP-3 908C, 0.1 MPa (CO2), 24 h 93 5 [59]

Epichlorohydrin COP-Al 908C, 1 MPa (CO2), 18 h, 0.57mol-% 98.1 5 [61]

Epichlorohydrin CCTF-350 1208C, 0.1 MPa (CO2), 24 h 97 5 [64]

Emerging Ionic Polymers for CO2 Conversion 775

moieties for improved CO2 capture and simultaneous

conversion� upgrading synthetic strategies to reduce the cost of PILs and

ILs

� extending the range of substrates (to more than epoxide) torealize the practical application of CO2 conversion into otherfine chemicals

� co-operating with computational and theoretical chemists to

understand the CO2 capture and catalytic mechanism, espe-cially with emerging task-specific ionic polymers containingdifferent functional groups

� building an encouraging environment between syntheticchemists and chemical engineers to pave the way for theintroduction of ionic polymer-based materials into the indus-

trial sector. This will shift the current proof-of-conceptapplications to more commercial applications.

To sum up, this review shows that emerging ionic polymersare being designed owing to the rich and available ionicchemistry coming from ILs and emerging approaches to pro-

duce porous polymers. Although several works have beenalready reported as shown here, we believe that the area is stillyoung and there is plenty of room for the design ofmore efficientcatalysts that allow CO2 conversion reactions to be carried out

more efficiently and at lower temperatures and pressures. It isworth remarking that although most of these polymers are beinginvestigated in CO2 conversion reactions, they also have many

possibilities as heterogeneous catalysts in other types of chemi-cal transformations.

Data Availability Statement

Data sharing is not applicable as no new data were generated oranalysed during this study.

Conflicts of Interest

The authors declare no conflicts of interest.

Declaration of Funding

D.S.S. thanks the Australian Research Council for fundingthrough a Future Fellowship (FT170100315). DM thanks theAgencia Espanola de Investigacion (AEI) for funding through

project PID2020–119026GB-I00. LCT is grateful to FCT(Fundacao para a Ciencia e a Tecnologia) in Portugal for herresearch contract under Scientific Employment Stimulus

(2020.01555.CEECIND). Associate Laboratory for GreenChemistry –LAQValso acknowledge the financial support fromFCT/MCTES (UIDB/50006/2020 and UIDP/50006/2020).

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Handling Editor: Curt Wentrup

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